Rotary drum vacuum filters are used in the chemical, pharmaceutical and pulp and paper industries to separate suspended solids from a slurry. Some applications, for example, are in the drying of effluent and sewage sludge. The function of the rotary drum vacuum filter is to remove the insoluble solids from the mother liquor, dry them, then form a cake or mat, and discharge them. The remaining mother liquor from which the insoluble solids have been removed is then discharged as filtrate. Depending on the application, the cake or mat can be washed using a weaker liquor while still on the drum prior to discharge. In applications where the suspended solids tend to settle in the vat, the solids are maintained in suspension in the vat by use of an agitator.
The rotary drum vacuum filter is constructed as a totally enclosed drum rotating partially submerged in a vat. Attached to the vat on one side is an inlet box which serves to collect the incoming slurry and permit uniform distribution into the main vat in which the drum is rotating. On the opposite side to the inlet box is a vat containing a repulper which receives the discharging cake or mat. The surface of the drum is covered with decking which supports a fine wire or synthetic mesh which serves to separate the suspended solids from the slurry. The decking can comprise wire winding, perforated plate, perforated plastic mat or corrugated stainless steel deck. The motive force for separation of the solids is normally a vacuum applied to the underside of the decking. This vacuum is provided either by a vacuum pump or, in the case of the pulp and paper industry where the solids are fibrous and therefore free draining, by a drop leg attached to the filter valve. Alternatively the drum/vat configuration can be totally enclosed by a hood which is pressurized. The decking is typically supported by longitudinal division bars together with intermediate bars. The division bars form ducts to carry the filtrate from beneath the decking and out of the drum through a valve or valves. The intermediate bars provide support for the decking and fine wire between the division bars and are perforated to permit the flow of filtrate across the shell surface or wrapper of the drum between the division bars.
The filtrate formed by the separation of solids from the mother liquor can be collected from underneath the decking in a variety of ways, but always exits by a valve, or sometimes two valves, located in the trunnion or at the rear end of the drum. A first common method of conveying the filtrate from the ducts between the division bars is to incorporate a ring of drainage buckets in the centre of the drum wrapper beneath the decking to collect the filtrate and then convey the filtrate to the valve or valves by means of pipes between the drainage bucket outlets and the valve itself. The number of pipes is equal to that of the number of longitudinal division bars and the pipes terminate in the valve area such that there is an individual outlet for each pipe. With this system, filtrate travels between the longitudinal division bars from each end of the drum before collection in the drainage buckets. In this way a stationary valve segment can be positioned in order to control which part of the filtration cycle requires the application of vacuum and in which part the pipe outlets should be blanked off corresponding to the portion of the cycle where vacuum is not required i.e. in the final phase of drying and where the cake or mat is removed.
A second common method involves end drainage. In the case of slow draining materials which produce a low volume of filtrate e.g. flocculated sewage sludge, it has been common to replace the internal piping concept by an end drainage system. According to this method, the filtrate passes longitudinally from one end of the filter surface to the other without the use of central drainage buckets. This design maintains the concept of drainage ducts incorporated in the drum surface bounded by the division bars. Instead of the filtrate flowing to the valve by internal piping, it exits through piping outside of the drum head directly into the filter valve. In recent years, such end drainage systems have been used for the pulp and paper industry for wood pulps having high, drainage rates. For slow draining pulps like bagasse, which exit small volumes of filtrate, the radial vertical pipe configuration outside the drum head is maintained. For fast draining wood pulps generating large volumes of mother liquor, the depth of the division bars is increased forming longitudinal ducts of uniform depth. The filtrate emerging from the longitudinal ducts passes though vertical, radial ducts which are incorporated in a segmented chamber inside the drum head to the valve area prior to discharge through the drop leg.
The range of rotational speeds required of vacuum filters varies widely depending on the application and can vary between 0.1 and 6 rpm. Various configurations of drive are used, some being trunnion mounted worm gear drives and others shaft mounted drives. In all cases, the speed of the filter can be varied, and different methods are used including, variable pulleys, hydraulic gears, variable speed DC motors and hydraulic motors.
The pulp mat must be discharged after the final drying part of the cycle and various devices are used to carry out this part of the operation. The simplest device for the take-off purpose is the metallic doctor blade, but this has the disadvantage of damaging the fine wire filtration medium. An improvement has been to raise the doctor blade from the surface of the drum and discharge a large volume of low pressure air between the tip of the doctor blade and the drum surface which lifts the pulp mat off the drum surface and onto the upper surface of the blade from whence it falls by gravity into the repulper conveyor. An alternative to this device is one where water is used instead of air. An earlier device was the fluted metallic roll which lifted the pulp mat off the drum surface onto an adjacent rubber covered roll from which the mat was removed by a metal doctor blade.
Both the pipe filter and end drainage designs of vacuum filter described above have inherent disadvantages. The ideal vacuum filter needs to exit filtrate from the drum internals in the shortest time possible. This is achieved by having the shortest residence time of filtrate possible inside the drum which is achieved by filtrate traveling through the filter internals by the shortest route possible and at constant velocity. There needs to be minimum turbulence in the flow of filtrate and the minimum of pressure drops which reduce the effectiveness of the vacuum applied. Pockets of air need to be avoided since these permit the exit of air entrained in the filtrate into the air pocket which is detrimental to the maintenance of vacuum. In the case of the conventional pipe filter described above, the filtrate is collected from both ends of the longitudinal duct between the division bars and the underside of the decking prior to collection in the centrally located drainage buckets. This means that half of the filtrate volume travels in one direction while the other half travels in the opposite direction, both meeting in the drainage bucket appropriate to that longitudinal duct the filtrate and creating an environment of extreme turbulence. Where the filter only has one valve, that half of the filtrate volume whose flow started from the valve end of the drum will need to reverse direction and flow in the direction from which it started prior to exiting the valve.
In the case of designs having segmented radial ducts feeding the filter valve which collect the filtrate emerging from the longitudinal ducts, because these ducts are radial, the cross-sectional area at the periphery is much greater than at the valve entry. This creates an undesirable condition, namely a large volume where air can escape from the filtrate forming a pocket which will reduce the effectiveness of the vacuum and promote non-uniformity in filtrate velocity.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.